W3C home > Mailing lists > Public > public-webrtc@w3.org > July 2013

Re: SDP is not suitable for WebRTC

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Tue, 30 Jul 2013 08:52:51 +1000
Message-ID: <CAHp8n2=D6jSmBGLtnhVYHj5FVKZ+X2x_KUM2ab1APVALMELQDQ@mail.gmail.com>
To: Iņaki Baz Castillo <ibc@aliax.net>
Cc: "rtcweb@ietf.org" <rtcweb@ietf.org>, "public-webrtc@w3.org" <public-webrtc@w3.org>
On Tue, Jul 30, 2013 at 3:31 AM, Iņaki Baz Castillo <ibc@aliax.net> wrote:
> Hi, I initiated a thread [*] about Plan-Unified and multiple m lines,
> but it was moved to MMUSIC maillist (don't know why since it is about
> WebRTC applications design):
> http://www.ietf.org/mail-archive/web/mmusic/current/msg11966.html
> Sorry for the cross-posting but at this point I'm a bit lost and do
> not know which is the appropriate group for my concern.
> So my concern is:
> - Web application with a SIP over WebSocket client running in the web.
> - The web user is provided with a conference SIP URI in which there
> are *already* 8 participants (5 of them emitting audio and video and 3
> just emitting audio).
> - The user calls, from his webphone, to the given URI to join the conference.
> Let's imagine that the JS app knows the number of participant in the conference.
> Let's imagine my browser have mic and webcam.
> How can my browser join the conference without requiring SDP
> renegotiation from the server and, at the same time, being able to
> send audio/video and receive audio/video from others (different tracks
> / m=lines)?
> 1)
> I tell my browser to generate a SDP offer with:
>   - 1 send/receive m=audio line.
>   - 7 recvonly m=audio line.
>   - 1 send/only m=video line.
>   - 4 recvonly m=video line.
> (Obviously this is a joke)

Since you seem to be doing a full mesh and you seem to be wanting to
receive from everyone and be able to talk back to everyone, wouldn't
this need to be 8 different SDP offers in a full mesh network rather
than one SDP offer with 13 different m-lines? I don't follow how each
of the recipients would pick the right m-line for themselves...

> 2)
> SDP seems to allow that the offer and the answer have different number
> of m lines (I'm not aware of that but I believe that SDP can do
> "everything"). So my browser generates a SDP offer with 1 m=audio line
> and 1 m=video line, and the server replies with 8 m=audio lines and 4
> m=video lines.
> Will my browser understand such a SDP answer with more m lines than
> its generated offer? I assume NOT.

I'm don't understand SDP, but logic would tell me that it would send a
single SDP offer with its own capabilities to each of the other 8
m=audio send/receive

(not sure if it also needs m=video, since it's not a video capable device IIUC)

> 3)
> My browser generates a SDP offer with 1 m=audio line and 1 m=video
> line and the server too. And later the server sends re-INVITE with all
> the m lines.
> Oppss, SDP renegotiation...

Yes, I would expect a negotiation to happen between each of the 8
current participants and the one new participant individually.
It would end up in 8 O/A negotiations that should all end up with 5
m=audio send/receive and 3 m=audio receive-only (from the POV of the

I'm taking a stab at this because I want to understand more about how
it works, but I actually have no idea if I am correct. Please,
somebody with SDP knowlege, clarify how this would actually work. I,
too, am very curious.

Received on Monday, 29 July 2013 22:53:37 UTC

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